1. Field of the Invention
The present invention relates to a method and device for measuring a nonlinear refractive index of an optical fiber, and in particular, to a method and device for measuring a nonlinear refractive index of an optical fiber which utilize a technique for rapidly measuring a nonlinear refractive index of an optical fiber with a simple structure.
2. Description of the Related Art
As is well known, an optical fiber is used as a transmission medium for transmitting light signals.
Because the optical fiber has a transmission loss in the same way as other transmission media, the longer the fiber length, the lower the strength of the light signal at the final end, the more the S/N deteriorates, and information cannot be accurately received. Therefore, there is the need to input a light signal having great strength at the inputting side.
However, the refractive index, which is an important factor determining the transmission characteristics of the optical fiber, exhibits dependency on the intensity of the light. The stronger the intensity of the light passing through, the more the refractive index increases.
This is called non-linearity of the refractive index of the optical fiber. The ratio of increase in the refractive index is called a nonlinear refractive index.
If light signal having great intensity is inputted to an optical fiber having a nonlinear refractive index, waveform distortion occurs in the light signal due to the nonlinear effect of refractive index. Adverse effects from the adjacent channel arise, and communication cannot be accurately carried out.
Accordingly, for example, when a communication system in which an optical fiber is the transmission medium is structured, there is the need to measure the nonlinear refractive index of the optical fiber in advance.
As a conventional method of measuring a nonlinear refractive index of an optical fiber, there are a method utilizing the self phase modulation effect of short pulse light, and a method utilizing the four-wave mixing effect by using two light sources.
The former method is a method in which short pulse light, whose strength is variable and which has a specific waveform, is incident on the optical fiber to be measured. The spectrum of the exiting light of the optical fiber is observed, and the inputting strength of the short pulse light is adjusted such that the number of peaks of the spectrum becomes a predetermined number. The peak power is determined by observing the time waveform of the short pulse light at this time. The nonlinear refractive index is determined on the basis of the peak power and the number of peaks of the spectrum.
Further, the latter method is a method in which two continuous lights having different frequencies (wavelengths) are merged and inputted to one end side of the optical fiber. The spectrum of exiting light of the optical fiber is observed. The ratio of the power of the two continuous lights and the power of two frequencies arising due to the four-wave mixing effect thereof is measured. The nonlinear refractive index is determined on the basis of the strength of inputted light and the power ratio.
However, there is the problems that, in the former measuring method, the measurement error becomes large by being affected by the frequency chirp (the change in frequency at the rise or fall of the pulse) or by the chromatic dispersion of the optical fiber, and in the latter measuring method as well, the measurement error becomes large by being affected by the chromatic dispersion of the optical fiber.
As a technique for solving this problem, for example, in Jpn. Pat. Appln. KOKAI Publication No. 8-285728, as shown in FIG. 18, a measuring method is proposed in which a nonlinear refractive index, in which the calculated result and the measured result sufficiently and precisely coincide, is determined by repeating, while changing a temporary value, a processing in which pulse light is incident on an optical fiber 1 which is a measuring object from a pulse light source 10, this incident light and the time waveform and frequency chirp characteristic of the exiting light of the optical fiber 1 are respectively measured by a time waveform measuring section 11 and a frequency chirp measuring section 12, the time waveform and the frequency chirp characteristic of the incident light are calculated in a calculating section 13 by numerical calculation of split-step Fourier method based on a nonlinear Schroedinger (Schrödinger) equation by using the time waveform obtained by measuring the incident light, the frequency chirp characteristic, known data of the optical fiber, and a temporary value of the nonlinear refractive index, and the calculated result and the actual measured result of the exiting light are compared.
However, in the above-described method disclosed in Jpn. Pat Appln. KOKAI Publication No. 8-285728, there is the need to precisely measure the time waveform, the frequency response characteristic chirp, and the power of the pulse light used as the measuring light. Therefore, there is the problem that an extremely high-speed light receiving device and a measuring circuit are necessary, and the device becomes expensive and large scale.
Further, in the above-described method disclosed in the Jpn. Pat. Appln. KOKAI Publication No. 8-285728, in the numerical calculation by the nonlinear Schroedinger equation for the pulse light, the calculating amount is great. Thus, there is the problem that the measured result cannot be rapidly obtained.